A display for displaying an image is one of the light-emitting elements indispensable in modern living. The display for displaying an image takes a variety of forms matched to applications, ranging from so-called a television monitor to a liquid crystal display rapidly developed in recent years and an organic EL display expected for future development. Particularly, a liquid crystal display and an organic EL display are light-emitting elements to be driven on low voltage, which are the most important image displays from the viewpoint of energy saving.
Among them, an organic EL display draws the greatest attentions as a flat panel display element in the next generation.
In the emission mechanism of an organic EL display, a thin film (hereinafter, described “organic thin film”) structured of a light-emitting body composition is provided between electrodes to flow current whereby the electrons injected from the cathode and the holes injected from the anode recombine at a luminescent center in a light-emitting body film and form a molecule exciton, to thereby utilize a photon released upon returning of the molecule exciton to the ground state.
Incidentally, the sort of the molecule exciton formed by the light-emitting body composition can be a singlet excitation state and a triplet excitation state. The present specification includes a case where any one of the excitation states contributes to light-emitting.
In such an organic EL display element (hereinafter, described “organic EL element”), an organic thin film is usually formed as a film as thin as below 1 μm. In addition, because a light-emitting body film itself is a self-light-emitting type element given out light, an organic EL element does not require a backlight as used on the conventional liquid-crystal display. Accordingly, it is a great merit that an organic EL element can be extremely fabricated thin and lightweight.
Moreover, for example in an organic thin film of nearly 100-200 nm, the time of from a carrier injection to a recombination reached is approximately several ten nanoseconds in the light of the carrier mobility in the light-emitting body composition film. Light-emitting is reached on the order of within a microsecond even if the course of from a carrier recombination to a light-emitting is included. Accordingly, very high-speed of response is also one of features.
Furthermore, because an organic EL element is a carrier-injection type light-emitting element, it can be driven on direct-current voltage, and hardly cause a noise. In addition, by forming a uniform ultra-thin organic thin film having nearly 100 nm in film thickness and using a suitable organic material, a driving is also possible on a voltage of several volts. Namely, an organic EL element, because of a self-light-emitting type and a wide in viewing angle, is well in visibility. Besides, an organic EL element also possesses the properties of thin and lightweight, high-speed responsibility, driving in direct-current and low voltage, and the like, and thus is expected as a light-emitting element in the next generation.
In order to fabricate such an organic EL element, there is an essential need of an art to form a thin film of a light-emitting body composition. In the liquid crystal display for example, in order to achieve a full-color display, there is a necessity to regularly form an organic thin film for functioning as a color filter on a glass substrate. On the other hand, in an organic EL element, a charge transport material for transporting the holes and electrons injected at an electrode and a luminescent material for light-emitting are a light-emitting body composition. These compounds must be formed with a filmy form between electrodes.
As techniques for forming such an organic thin film, various methods have been developed including a Langmuir-Blrogett method (LB method), a monomolecular film stack method, a dip coating method, a spin coat method, an inkjet method, a print method, a evaporation method, or the like. Among them, an inkjet method has particularly a merit that an organic material can be used with high-efficiency, a configuration of an apparatus is simple and can be reduced in size, and so on. Technically, it is already approximated to the practical application level. The basic technology concerning an inkjet method is disclosed in Patent Document 1, etc.
Patent document 1: Japanese Patent Laid-Open No. H10-12377
An inkjet method is an art that an inkjet system employed on the conventional printer is converted to a thin film forming, which is a method to apply droplets on a pixel to pixel basis by using, in place of ink, a solution or a dispersion liquid containing a light-emitting body composition as a material of an organic thin film. By volatilizing the solvent contained in the droplet, a thin film is formed on the individual pixel. By controlling the position of the droplet attached on a substrate, it is possible to form an arbitrary micropattern.
However, because the droplet deposited on the pixel (actually, pixel electrode provided in each pixel) contains a great amount of solvent component, there is a need of a process for volatilizing a solvent component (hereinafter, described “baking process”) in order to remove a solvent component. Namely, after applying a droplet by an inkjet method, the pixel entirety is heated to volatilize a solvent component and thereby the remaining solute (material of an organic thin film) is formed with a thin film. Accordingly, in a case where the solvent of the solution containing a light-emitting body composition has a low vapor pressure, time is required in the baking process. Besides, a droplets attached on the neighboring pixels is ready to mix together, and the formation of a microscopic thin film pattern is hindered. In addition, when a solvent component is left in the thin film, the solvent volatilizes with time and a degasification phenomenon is caused. Therefore, a factor incurring a deterioration in the organic thin film and ultimately a deterioration as a light-emitting element is caused. Furthermore, if the heating temperature is raised to remove the solvent component completely, it results in destruction in the composition of an organic thin film having a low heat resistance.
In this manner, a formation method of the organic thin film based on ink jet method is advantageous in low cost and simple. However, a formation method of the organic thin film based on ink jet method has a problem in baking process, and thus is an art left room for improvement.